This invention relates to the comminution of solid material and, more particularly, is concerned with the comminution of solid materials comprising calcium carbonate.
It is known that the product obtained on comminuting a solid material comprises particles of sizes which vary over a wide range and which follow a distribution curve which approximates to a Gaussian distribution. As a result, if it is desired to comminute a solid material to produce a product substantially all of which consists of particles smaller than a given size, the product will inevitably contain a large proportion of particles which are very much finer than the given size and many particles which may be undesirably fine for the purpose for which the ground product is required. Also a considerable quantity of energy is wasted in the comminution process in producing the large proportion of particles which are very much finer than is required. The problems outlined above may be overcome by subjecting the product of the comminution process to a particle size classification step, the fine product of the classification step being returned for further grinding. In this manner it is possible to avoid the production of large proportions of undesirably fine material and the consumption of large amounts of energy in producing a product substantially all of which consists of particles smaller than a given size. When, however, the solid material to be comminuted comprises calcium carbonate problems arise in practising the process of comminution followed by a particle size classification step as outlined above.
Much ground calcium carbonate material is used as a pigment in paper coating compositions and for this purpose is required in the form of a highly concentrated, deflocculated aqueous suspension having a solids content ranging from about 65 to about 80% by weight. Since natural calcium carbonate occurs in a dry or substantially dry state (i.e. containing not more than about 20% by weight of water) it is convenient to comminute the calcium carbonate in the form of a high solids content, deflocculated aqueous suspension as is described, for example, in U.S. Pat. Nos. 3,604,634 and 3,995,817 and in U.S. Pat. application Ser. No. 73,880, filed Sept. 10, 1979, as a continuation of Ser. No. 880,875, now abandoned. The product obtained by comminuting at a high solids concentration in this manner may be subjected to a particle size classification by means of certain special types of centrifuge, as is described in U.S. Patent Application Ser. No. 927,360, but such centrifuges tend to be costly and a considerable amount of energy is required to remove from the centrifuge the extremely viscous coarse product of the classification.
An alternative to comminuting and classifying a calcium carbonate material as a high solids content suspension is to perform these operations at a much lower solids concentration, but this has the disadvantage that water must be added to the suspension of calcium carbonate material in order to form the low solids suspension and then removed from the ground product to bring the latter into a saleable form. Also, if the suspension is to be dewatered by filtration or centrifugation it is essential for the formation of a good dewatered product of relatively low water content that the suspension should be in the flocculated state. This generally means that a chemical flocculant must be added, and this increases the cost of the process. Also, it has been found that it is often extremely difficult to redisperse or deflocculate again a finely ground natural calcium carbonate which has previously been flocculated in the conventional manner.
In my U.S. Patent Application Ser. No. 865,242 now U.S. Pat. No. 4,166,582, I have disclosed a method of comminuting a solid material comprising calcium carbonate to obtain a comminuted solid material containing at least 60% by weight of particles smaller than 2 microns equivalent spherical diameter, in which method (a) an aqueous suspension of the solid material is formed having a solids content in the range of from 5% to 50% by weight of dry solids and containing sufficient dispersing agent to prevent the formation of agglomerates during the subsequent comminution of the solid material; (b) the solid material in the suspension formed in step (a) is comminuted by agitating the suspension in admixture with a particulate grinding medium which consists of particles not larger than about 10 mm and not smaller than about 0.15 mm; (c) an aqueous suspension containing comminuted solid material at least 60% by weight of which is smaller than 2 microns equivalent spherical diameter is separated from the product of step (b); (d) the comminuted solid material in the separated aqueous suspension is flocculated (if not already in a flocculated condition) by means of an electrolyte having a multivalent cation; and (e) the aqueous suspension containing the comminuted and flocculated solid material is dewatered. This method makes it possible to comminute a solid material comprising calcium carbonate in an aqueous suspension at a relatively low solids content and to flocculate and dewater the suspension in such a manner that the dewatered and comminuted solid material may be deflocculated easily and at the expense of only a small quantity of dispersing agent.
I have now found that it is possible to incorporate into the method described in my U.S. Patent Application Ser. No. 865,242, now U.S. Pat. No. 4,166,582 an additional particle size classification step in order to save energy and to avoid the production of a large proportion of undesirably fine particles.